A multipoint study of a substorm occurring on 7 December, 1992, and its theoretical implications

N. J. Fox1, S. W. H. Cowley2, V. N. Davda3, G. Enno4, E. Friis-Christensen5, R. A. Greenwald6, M. R. Hairston7, M. Lester2, M. Lockwood3, H. Lühr8, D. K. Milling9, J. S. Murphree4, M. Pinnock10, and G. D. Reeves111Laboratory for Extraterrestrial Physics, NASA/GSFC, Greenbelt, MD 20771, USA
E-mail: nicola.fox@gsfc.nasa.gov2Physics Department, University of Leicester, LE1 7RH, UK3Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK4Department of Physics and Astronomy, University of Calgary, Alberta, Canada T2N 1N45Danish Space Research Institute, Juliane Marie Verj 30, DK-2100 Copenhagen, Denmark6John Hopkins University, Applied Physics Laboratory, Laurel, MD 20723, USA7University of Texas at Dallas, Richardson, TX 75080, USA8GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany9Department of Physics, University of York, York YO1 5DD, UK10British Antarctic Survey, Cambridge CB3 0ET, UK11Los Alamos National Laboratory, Los Alamos, NM 87545, USAAbstract. On 7 December 1992, a moderate substorm was observed by a variety of satellites and ground-based instruments. Ionospheric flows were monitored near dusk by the Goose Bay HF radar and near midnight by the EISCAT radar. The observed flows are compared here with magnetometer observations by the IMAGE array in Scandinavia and the two Greenland chains, the auroral distribution observed by Freja and the substorm cycle observations by the SABRE radar, the SAMNET magnetometer array and LANL geosynchronous satellites. Data from Galileo Earth-encounter II are used to estimate the IMF Bz component. The data presented show that the substorm onset electrojet at midnight was confined to closed field lines equatorward of the pre-existing convection reversal boundaries observed in the dusk and midnight regions. No evidence of substantial closure of open flux was detected following this substorm onset. Indeed the convection reversal boundary on the duskside continued to expand equatorward after onset due to the continued presence of strong southward IMF, such that growth and expansion phase features were simultaneously present. Clear indications of closure of open flux were not observed until a subsequent substorm intensification 25 min after the initial onset. After this time, the substorm auroral bulge in the nightside hours propagated well poleward of the pre-existing convection reversal boundary, and strong flow perturbations were observed by the Goose Bay radar, indicative of flows driven by reconnection in the tail.